Recently, with exhaustion of fossil energy and concern about environmental pollution, there is an increasing interest in electric products operating using electrical energy, but not using fossil energy.
Hence, with the increasing development and consumption of mobile devices, electric vehicles, hybrid vehicles, power storage systems, uninterrupted power supplies, or the like, the consumption of secondary batteries as energy sources are drastically increasing. Particularly, secondary batteries in use for electric vehicles or hybrid vehicles are high output and large capacity secondary batteries, and studies are actively being conducted thereon.
Currently, common secondary batteries include nickel-cadmium batteries, nickel-hydrogen batteries, nickel-zinc batteries, lithium secondary batteries, and the like, and particularly, lithium secondary batteries have a high energy density per unit weight when compared to other secondary batteries such as existing lead storage batteries, nickel-cadmium batteries, nickel-hydrogen batteries, and nickel-zinc batteries, and allows rapid charging, and thus, they are being increasingly used. Also, when compared to nickel-cadmium batteries or nickel-metal hydride batteries, lithium secondary batteries have triple operating voltage and excellent characteristics in terms of energy density per unit weight, and for these reasons, are being increasingly used.
A lithium secondary battery may be categorized into a lithium ion battery using a liquid electrolyte and a lithium ion polymer battery using a polymer solid electrolyte, based on an electrolyte type. Also, a secondary battery may be classified into a pouch-type secondary battery, a cylindrical secondary battery, and a prismatic secondary battery based on an outer case type.
A pouch-type secondary battery, a type of a secondary battery, includes a pouch-shaped case made of an aluminum laminate sheet, and a cell assembly including a stack of plural electrochemical cells, each including a cathode plate/separation film/anode plate, received in the pouch-shaped case. This pouch-type secondary battery has advantages of low battery fabrication costs, a remarkably reduced weight, and easy shape adaptation over a can-type secondary battery.
However, a pouch-type secondary battery is very vulnerable to high temperature. That is, when a pouch-type secondary battery is overheated, gas is generated inside and an outer case or pouch-shaped case swells up. When swelling comes to extremes, the secondary battery may explode. Also, when the temperature of the secondary battery drastically increases due to a short circuit current, gas generated in the outer case ignites, causing explosion accompanied by a fire accident.
Conventionally, to prevent a secondary battery from being overheated, a protection device that measures a temperature change of a secondary battery, and stops charging and discharging of the secondary battery immediately when temperature excessively increases, has been widely used.
Such a conventional secondary battery protection device measures a surface temperature of a secondary battery, to be specific, a surface temperature of a pouch-shaped case, and monitors the measurements. When a temperature change of a secondary battery is monitored in this way, unfortunately, follow-up measures can be taken but precautionary measures are virtually impossible.
That is, a representative cause of a drastic temperature increase of a secondary battery is a flow of short circuit current. A short circuit current is primarily caused by a short circuit occurring in a secondary battery due to penetration of a needle-shape object or the like, or a short circuit occurring in an electronic device connected to the secondary battery or the like.
When a short circuit phenomenon occurs in a secondary battery, a radical electrochemical reaction occurs between a cathode plate and an anode plate and thereby heat is generated. The generated heat passes to a surrounding material, and by this heat transfer, a surface temperature of a pouch-shaped outer case rises at a high speed.
In consideration of heat generation and a heat transfer mechanism, there is unavoidably a predetermined time difference from an occurrence time of a short circuit phenomenon to a start time of a drastic increase in surface temperature of a pouch-shaped outer case. This is because it takes some time to transfer heat generated between a cathode plate and an anode plate of a secondary battery to a surface of a pouch-shaped outer case.
Thus, a point in time at which a secondary battery protection device detects overheat of a pouch-shaped outer case comes only after a problem with safety of the secondary battery occurred due to a flow of short circuit current for a considerate amount of time. To solve this problem, there is a need for a method for detecting an abnormal temperature change quickly and correctly by directly measuring an internal temperature of a secondary battery.
Meanwhile, gas is generated by a chemical reaction in a battery case during charging and discharging of a secondary battery, and the generated gas applies pressure between electrode plates in a close contact, causing a loose phenomenon of the electrode plates. This loose phenomenon of the electrode plates leads to a loss in the viewpoint of energy efficiency of the secondary battery.
Accordingly, there is an urgent need for development of a secondary battery having a structure for preventing a loose phenomenon caused by gas generation while measuring a temperature change of a secondary battery correctly and quickly.